Semiconductor lasers used as light sources of optical information processing devices should have long life, not vary widely in threshold current with temperature, and produce little noise. The semiconductor lasers used in the optical information processing devices can be ridge-type semiconductor lasers which have an oscillation wavelength in the red range and includes a current block layer of a double layer structure and the like (for example, see Patent Literature 1).
In a light information processing device including a single-mode semiconductor laser, if a laser beam reflected on an optical disk or the like is incident on the semiconductor laser, the oscillation unstably changes because of the light interference, causing noise. Such noise due to the returned light works against using the semiconductor lasers in light sources for reproduction and recording of optical disks or the like.
In order to reduce the noise due to the returned light, generally, the coherence of the laser light is reduced by modulating the output of the semiconductor laser with a high-frequency superposing circuit. However, this method additionally requires the high-frequency superposing circuit for generating high-frequency current and is not suited for miniaturization of a light information processing device on which the semiconductor laser is mounted. Recently, a so-called self-pulsation laser which periodically changes in output without the extra high-frequency superposing circuit has attracted attention because the self-pulsation laser is suited for miniaturization and is produced at low cost.
However, the self-pulsation laser has a problem of operating as a single mode laser at high temperature. In other words, the conventional self-pulsation laser is guaranteed to have the same effect as that obtained by superposing high frequency current onto a single mode laser only when the laser operates at low temperature.
Patent Citation 1: Japanese Patent Laid-open Publication No. 2002-124736